Disk drive comprising a double sided flex circuit wherein a first side lead provides an etching mask for a second side lead

ABSTRACT

A method of manufacturing a flex circuit is disclosed for a disk drive comprising a disk, a head actuated radially over the disk, and control circuitry. The flex circuit is for electrically coupling the head to the control circuitry and comprises a substrate. An electrical coating applied to a first side of the substrate is etched to form a first electrical lead. The first side of the substrate is irradiated with radiation such that the first electrical lead masks the radiation from passing through the substrate to prevent curing of a photoresist applied to the second side of the substrate to form an uncured photoresist and a cured photoresist on the second side of the substrate. The uncured photoresist is removed from the second side of the substrate to form a groove, and the groove is filled with electrically conductive material to form the second electrical lead.

BACKGROUND

FIG. 1A shows a prior art disk drive comprising a disk 2 and a head 4 connected to a distal end of an actuator arm 6 which is rotated about a pivot by a voice coil motor (VCM) 8 to position the head 4 radially over the disk 2. The head 4 may comprise an inductive write element (write coil) and a magnetoresistive read element (MR element) fabricated in very small dimensions using semiconductor fabrication techniques. A flex circuit 10 is typically employed to electrically couple the head 4 to control circuitry within the disk drive. In the example shown in FIG. 1A, a first flex circuit 10A couples the head 4 to a preamp 12A mounted on the actuator arm 6, and a second flex circuit 10B couples the preamp 12A to other control circuitry 12B mounted on a printed circuit board, wherein the second flex circuit 10B facilitates the movement of the actuator arm 6. In other disk drives, the preamp 12A may be integrated with control circuitry 12B such that flex circuit 10A couples the head 4 directly to the control circuitry 12B mounted on the printed circuit board.

FIG. 1B shows a magnified cross-sectional view of the flex circuit 10A as comprising electrical leads for carrying differential signals, such as a differential write signal 14A and 14B and a differential read signal 16A and 16B for the head 4. The electrical leads are supported by a substrate 18 which may comprise any suitable material, such as a polyimide. The electrical leads are typically formed using conventional etching techniques on one side of the substrate 18 such that the electrical leads for carrying the differential signal are separated by an air gap (e.g., air gap 20A and 20B).

As the data rate in disk drives increases into the microwave region, the transmission properties of the electrical leads for carrying the differential signals has become more significant. For example, it is desirable to reduce the impedance of the electrical leads in order to increase power efficiency as well as the signal-to-noise ratio (SNR) of the differential signal.

There is, therefore, a need in a disk drive to reduce the impedance of the electrical leads fabricated on a flex circuit in order to improve the power efficiency and SNR in transmitting differential signals along the electrical leads.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a prior art disk drive comprising a head actuated over a disk and a flex circuit for coupling the head to control circuitry.

FIG. 1B shows a prior art flex circuit comprising electrical leads for carrying differential signals fabricated on a single side of a substrate.

FIG. 2A shows a disk drive according to an embodiment of the present invention comprising a head actuated over a disk and a flex circuit for coupling the head to control circuitry.

FIG. 2B shows a flex circuit according to an embodiment of the present invention wherein the electrical leads for carrying differential signals are fabricated on opposite sides of a substrate, wherein a first electrical lead provides an etching mask for etching the second electrical lead.

FIGS. 3A-3H show a method of manufacturing the flex circuit according to an embodiment of the present invention wherein a first electrical lead provides an etching mask for etching the second electrical lead.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 2A shows a disk drive according to an embodiment of the present invention including a disk 22, a head 24 actuated radially over the disk 22, control circuitry 26A and 26B, and a flex circuit 28A for electrically coupling the head 24 to the control circuitry 26A (a preamp in the example shown in FIG. 2A). The flex circuit 28A (FIG. 2B) comprises a substrate 30, a first electrical lead 32A coupled to a first side of the substrate 30, wherein the first electrical lead 32A is operable to conduct a first signal of a differential signal, and a second electrical lead 32B coupled to a second side of the substrate 30 opposite the first side, wherein the second electrical lead 32B is operable to conduct a second signal of the differential signal. The first electrical lead 32A provides an etching mask for etching the second electrical lead 32B, and the first electrical lead 32A is substantially aligned with the second electrical lead 32B such that the substrate 30 forms a capacitive dielectric.

In the embodiment of FIG. 2A, the head 24 is connected to a distal end of an actuator arm 36 which is rotated about a pivot by a voice coil motor 38 in order to actuate the head 24 radially over the disk 22. A first flex circuit 28A couples the head 24 to a preamp 26A mounted on the actuator arm 36, and a second flex circuit 28B couples the preamp 26A to other control circuitry 26B mounted on a printed circuit board. As the actuator arm 36 rotates, the second flex circuit 28B bends to facilitate the movement of the actuator arm 36. In an alternative embodiment, the preamp 26A is integrated with the other control circuitry 26B such that flex circuit 28A couples the head 24 directly to the control circuitry 26B mounted on the printed circuit board.

The flex circuit 28A may comprise electrical leads for carrying any suitable differential signal. In one embodiment, the head 24 comprises a magnetoresistive (MR) head comprising a write element having a first differential signal interface (e.g., 32A and 32B) and a read element having a second differential signal interface (e.g., 34A and 34B). As described above, it is desirable to reduce the impedance of the electrical leads carrying a differential signal in order to increase power efficiency as well as the signal-to-noise ratio (SNR) of the signals. The impedance can be reduced by increasing the capacitance between the electrical leads, and in the embodiment shown in FIG. 2B, the impedance is reduced due to the increased capacitance of the substrate 30. However, in order to take full advantage of the capacitive dielectric property of the substrate 30, in one embodiment the first electrical lead (e.g., 32A) is substantially aligned with the second electrical lead (e.g., 32B).

FIGS. 3A-3H show a method of manufacturing the flex circuit 28A according to an embodiment of the present invention so that the electrical leads carrying a differential signal are substantially aligned. Referring to FIG. 3A, an electrical coating 40 is applied to a first surface of a suitable substrate 30 (e.g., a polymide), wherein the electrical coating 40 may comprise any suitable material, such as a metal alloy comprising copper, beryllium copper, nickel, or compositions thereof. A suitable photoresist 42 (e.g., a suitable polymer) is applied over the electrical coating 40, and a mask 44 is placed over the photoresist 42 (FIG. 3B). A suitable radiation source (e.g., ultraviolet light or visible light) is directed at the first surface so as to cure the photoresist 42 not covered by the mask 44. Referring to FIG. 3C, the uncured photo resist 42 and underlying electrical coating 40 are removed (etched) using a suitable etchant solution, such as acid ferric chloride. The cured photoresist 42 shown in FIG. 3C is then removed (FIG. 3D) using a suitable solution, such as an organic solvent (e.g., methylene chloride), leaving the first electrical lead 32A shown in FIG. 2B.

During the step of etching the electrical coating 40 applied to the first side of the substrate 30 to form the first electrical lead 32A as described above with reference to FIGS. 3A-3D the mask 44 may be inverted if a positive photoresist 42 is employed. In this embodiment, the masked part of the photoresist 42 is cured when developed and the unmasked (and uncured) photoresist 42 is removed together with the underlying electrical coating 40 as shown in FIG. 3C.

Continuing now with FIG. 3E, the substrate 30 is flipped over so that the second side is facing up, and a photoresist 46 is applied to the second side. The first side of the substrate 30 is then irradiated as shown in FIG. 3E such that the first electrical lead 32A masks the radiation from passing through the substrate 30 to prevent curing of the photoresist 46 applied to the second side of the substrate 30, thereby forming an uncured photoresist and a cured photoresist on the second side of the substrate. In this embodiment, the substrate 30 is sufficiently transparent to pass the radiation, whereas the first lead 32A masks the radiation. Referring to FIG. 3F, the uncured photoresist 46 is removed from the second side of the substrate 30 to form a groove 48. Referring to FIG. 3G, the groove 48 is filled with electrically conductive material 50 using any suitable technique, such as a suitable deposition process (e.g., a liquid bath plating process or sputtering process). The cured photoresist 46 shown in FIG. 3G is then removed as shown in FIG. 3H, thereby forming the second electrical lead 32B shown in FIG. 2B.

As seen in FIG. 3H, the first electrical lead 32A is substantially aligned with the second electrical lead 32B such that the substrate 30 forms a capacitive dielectric. In one embodiment, the capacitive dielectric of the substrate 30 increases the capacitance of the electrical leads 32A and 32B as compared to the air dielectric shown in the prior art of FIG. 1B. Increasing the capacitance reduces the impedance of the electrical leads in order to increase power efficiency as well as the signal-to-noise ratio (SNR) of the differential signals. 

1. A method of manufacturing a flex circuit for a disk drive, the disk drive comprising a disk, a head actuated radially over the disk, and control circuitry, wherein the flex circuit is for electrically coupling the head to the control circuitry and comprises a substrate, the method comprising: etching an electrical coating applied to a first side of the substrate to form a first electrical lead; irradiating the first side of the substrate with radiation such that the first electrical lead masks the radiation from passing through the substrate to prevent curing of a photoresist applied to the second side of the substrate to form an uncured photoresist and a cured photoresist on the second side of the substrate; removing the uncured photoresist from the second side of the substrate to form a groove; and filling the groove with electrically conductive material to form the second electrical lead.
 2. The method as recited in claim 1, wherein the first electrical lead is substantially aligned with the second electrical lead such that the substrate forms a capacitive dielectric.
 3. The method as recited in claim 1, wherein the substrate comprises a polyimide.
 4. The method as recited in claim 3, wherein the polyimide is sufficiently transparent to pass the radiation.
 5. The method as recited in claim 1, wherein the radiation comprises ultraviolet light.
 6. The method as recited in claim 1, wherein the radiation comprises visible light.
 7. A disk drive comprising: a disk; a head actuated radially over the disk; control circuitry; and a flex circuit for electrically coupling the head to the control circuitry, the flex circuit comprising: a substrate; a first electrical lead coupled to a first side of the substrate, wherein the first electrical lead is operable to conduct a first signal of a differential signal; a second electrical lead coupled to a second side of the substrate opposite the first side, wherein the second electrical lead is operable to conduct a second signal of the differential signal; and wherein the first electrical lead provides an etching mask for etching the second electrical lead and the first electrical lead is substantially aligned with the second electrical lead such that the substrate forms a capacitive dielectric.
 8. The disk drive as recited in claim 7, wherein the substrate comprises a polyimide.
 9. The disk drive as recited in claim 8, wherein the polyimide is sufficiently transparent to pass a curing radiation.
 10. The disk drive as recited in claim 7, wherein the radiation comprises ultraviolet light.
 11. The disk drive as recited in claim 7, wherein the radiation comprises visible light.
 12. The disk drive as recited in claim 7, further comprising an actuator arm, wherein: the head is coupled to a distal end of the actuator arm; the control circuitry comprises a preamp circuit mounted on the actuator arm; and the flex circuit for electrically coupling the head to the preamp circuit.
 13. The disk drive as recited in claim 7, wherein the control circuitry is mounted on a printed circuit board. 